Proprietary is a trait of any resource which is provided by an owner who retains one or more exclusive legal rights within a specific social context.
Use of the term proprietary has almost always seemed to imply the exclusively retained legal right to require payment in money for others' use of the resource in question. However, it's debatable whether or not that right should be deeply separated from all other exclusive legal rights. (Note that some open licenses create intentionally inclusive rights which don't create or support any restrictions.)